This invention relates to a method and apparatus to produce an electric antiskid control signal for each braked wheel of motor vehicles and more particular to such apparatus where a reference value is produced in accordance with the state of motion of each braked wheel and comparing this reference value with the continuously instantaneously measured actual value of the state of motion of each braked wheel to provide a control signal for each braked wheel actuate an associated correcting element for controlling the effective brake pressure at each braked wheel.
Every driver knows the undesired and dangerous attendant circumstances of the locking of all or separate vehicle wheels. When the brake force applied by the driver can no longer be transmitted to the roadway, for example, because of an emergency braking or simply because of a too low frictional value between the tire and the roadway, the respective wheel is locked. This results in an increase in the stopping distance since the friction value between the gliding surfaces decreases more. When the front wheels are locked, the vehicle can no longer be steered, and when the rear wheels are locked, the vehicle can skid. Experienced drivers oppose these actions by braking intermittently, i.e. they apply the brakes and release them again, so that the respective wheel -- when the wheel tends to lock when the brakes are applied -- is shortly afterwards accelerated again and then it is again braked. In this way the vehicle is prevented from gliding over the roadway without the driver being able to steer the vehicle or that the vehicle will rotate about its vertical axis. However, an optimum braking cannot be reached. The increasing speeds of motor vehicles and the increasing density of traffic have long since rendered an automatic antiskid control necessary for the future. The stopping distance has to be reduced to a minimum, the steering capacity of the vehicle has to be maintained and a breaking away of the vehicle has to be prevented.
It would be ideal if the slip, i.e. the difference between the vehicle speed and the wheel speed, could be made the control signal of a preferably electronically, hydraulically operating antiskid control system.
As is well known the friction value between the roadway and the wheel at first strongly increases with increasing slip, then reaches a maximum at about 10 to 20% slip and then drops more or less abruptly. Apart from the friction between the wheel and the roadway, this maximum is also dependent upon the momentary wheel speed. This has rarely been taken into account in arrangements for an antiskid control system made up to now.
For example, there has been a search for a method of measuring the slip of the vehicle wheels and to hold the slip at a value of 10 to 20% by means of correspondingly controlling the effective brake pressure in order to reach an optimum braking. However, even the measurement of the vehicle speed during the braking process caused considerable difficulties.
With the brake control system described in the German Patent specification No. 2,051,899 laid open for public inspection, the attempt was made to apply this method. Therein it is suggested that to approximate the vehicle speed first the acceleration of the vehicle is presented in the form of an electrical voltage by means of a known inertia acceleration sensor and then integrating this value to determine the vehicle speed. Errors occur with this method and should be corrected by comparing the signal produced by the integration slip with the signal of the speed sensor of a wheel which is intermittingly freely rolling during braking. When there is a difference between these two signals, the signal which is produced by integration will be corrected by the signal of the speed sensor. This signal, representing the speed of the vehicle, is transmitted in the electronic unit to a number of circuits in order to be compared with the signal representing the momentary speed of each of the vehicle wheels and to be processed into a value representing the slip of the associated wheel. If the slip exceeds a certain threshold value, the electronic controller transmits an actuating signal to the correcting element, for example, a solenoid valve, to reduce the effective brake pressure up to the point when the state of motion of the wheel has recovered, i.e. the threshold value is exceeded in the opposite direction. At this point the actuation signal alters its sign and via the correcting element initiates an increase of the brake pressure. This process can be repeated, so that the deceleration of the wheel oscillates in a slip range which is previously determined.
However, this above described arrangement does not solve all the difficulties since during each braking the reference wheel has to be cyclically released so that it can become freely rolling and attain the actual vehicle speed. By releasing the reference wheel the stopping distance certainly cannot be optimally short. Also side forces can occur so that the vehicle can drift to one side. It is also a disadvantage that the reference value for the control of all vehicle wheels is determined by the action of only one single reference wheel. When this reference wheel has, for example, during braking a considerably worse friction value than the other wheels, the reference wheel can lock during its braking and it needs, when it is temporarily released, a longer period of time for rolling to reach the vehicle speed again. Therefore, the period of release, which must not be too long because of the increase of the stopping distance, possible is not sufficient and, thus, an adulteration of the reference signal occurs. Also the inertia acceleration sensor whose integrated output signal represents the basis for the reference signal, is, for instance, on inclinations subject to disturbances which have to be compensated for prior to integration.
Other proposals which use the wheel acceleration as a control value, also can only offer an approach to the actually desired brake course. It should be taken into account that the wheel acceleration is dependent on other factors like speed, friction value and other factors. Disturbances which are caused by bumps in the roadway and oscillations of the vehicle have to be eliminated. In order to reach an approach as good as possible to the desired brake course several acceleration thresholds have to be introduced as switching values.